How to add NULL condition on Auth::attempt

I've overwritten AuthenticateUsers::credentials to add a condition, that works, but I'd like to check for a NULL value in a column on my users table, so I've tried this

protected function credentials(Request $request)
{
    return array_merge(
        $request->only($this->username(), 'password'),
        ['my-column' => null]
    );
}

But that bounces me back when attempting to login